winscape 2239

The BIOS basically is the boot rom plus some hooks for video-comm system calls. They were invented back in the old days of the 8086-8088 when the old IBM PC had BASICA in rom -- so you could run basic programs and such without even a disk operating system.

Anyway, there's a number of system call like things in the bios along with the boot rom stuff. Self test lives in there along with disk recognition... memory test. Stuff pdp11's used in the boot loader along with some of what was in XXDP for diagnosing problems.

Here's how stuff through the 486 booted...

The power supply turns on... if the motherboard sends up a power ok signal back it doesnt do a poweroff.

The machine powers up, and jumps to a memory segment mapped into the rom.. This is normally location FFFF0h, right at the end of the system memory. This location has a jump into the real BIOS power on self test (POST).

The Bios looks for the video card and runs it's bios test (the BIOS for the video is normally at C000h. The bios looks for any additional controllers with their own bios initialization rom... and runs those. The hard disk controller was usually at C800h...

If all is OK the bios tests for serial and parallel ports and puts the addresses of those in the memory at 0000:40 (IIRC). That's why you can just use debug under DOS to rearrange your serial ports and swap them.

Plug and play happens next...

The bios displays a config screen and looks for where to boot from and loads the boot loader and then the os. (On hard disk it looks for the master boot record at Cyl 0 Head 0 Sector 1 and then uses the boot sector to boot the OS.

more details at:

details of the boot process, master boot sector and boot loader are here.

Bill

Bill

-- -- digital had it THEN. Don't you wish you could still buy it now! pechter-at-ureach.com